I flattened a 48" wide by 72" long slab two-three weeks ago with hand planes (I'm the only professional in my area that does this as far as I'm aware). It was a 5 hour job. I
You'll have to joint by hand for the finished edges. What I would do is to buy rough lumber, have it machine jointed on one face and two edges. Hand plane, glue up into wide sections (16") have someone with a wide machine joint and plane those sections, then glue those two sections in place and hand plane the result.
OK, good luck.